The invention relates to a projection screen, in particular for a color-television projection system, which screen comprises a single plate of a transparent material having a first and a second surface and provided with light diffusers, the second surface being intended to be directed towards an audience. The invention also relates to a projection system employing such a projection screen and to a method of manufacturing the screen.
Such a projection screen for the reproduction of a picture projected by an image source may be employed for the projection, generally on an enlarged scale, of films, commercials or television pictures and is described inter alia in European Patent Application No. 0,051,977 to which U.S. Pat. No. 4,443,814 corresponds.
The quality of a picture displayed by means of a projection system largely depends on the optical properties of the projection screen used. The picture formed by means of such a projection screen must be such that a large viewing range is obtained in the horizontal direction. In the vertical direction, the viewing range is generally smaller. The brightness distribution must be as uniform as possible over the entire picture regardless of the angle at which the picture is viewed by the audience. When the screen is used in a color-television projection arrangement employing three in-line cathode ray tubes for the primary colors red, green and blue a change in position of the viewer or a change in viewing angle should not lead to color-shifting in spite of the fact that the three beams which are projected at the same point are incident at different angles. Finally, the picture must have maximal brightness and maximal contrast.
The light diffusion necessary for a large horizontal viewing range can be obtained by means of a structure of adjacent cylindrical lenses whose cylinder axes extend in a vertical direction, which structure is provided on one surface of the screen. This always requires use of a Fresnel lens structure in order to ensure that the light from the image source falls onto the screen substantially perpendicularly. This Fresnel lens may be arranged on the rear surface of the projection screen, i.e. the surface which faces the image source. The advantage of such a single element screen is that it is comparatively inexpensive. However, this screen also has some disadvantages. Firstly, interference of the Fresnel lens with said cylindrical-lens structure gives rise to Moire effects. Furthermore, the picture obtained by means of the projection screen exhibits poor contrast because the front of the screen reflects much ambient light towards the audience. Moreover, one of the surfaces is generally matt, giving rise to a further deterioration of the contrast. Finally, color faults may arise in the case of three-color projection with separate projection axes for these colors.
A substantial improvement in contrast can be achieved by arranging the horizontally diffusing lens structure on the rear surface of the screen and by arranging absorbing strips on the front of the screen outside the focal lines formed by the lens structure. The Fresnel lens must then be arranged on a separate carrier. One surface of this carrier may be matt to obtain some light diffusion in the vertical direction.
The disadvantages of this two-element projection screen are that Moire effects may occur as a result of interference between the vertical cylindrical-lens structure and the Fresnel lens and that the price is comparatively high because of the use of two plates.
The afore-mentioned European Patent Application No. 0,051,977, describes a projection screen for color-television projection comprising a single plate. Both surfaces of this plate are formed by a two-dimensional lenticular structure. The corresponding lenses on the front and the rear of the plate have a common or a substantially common optical axis, the focal point of each lens on the rear of the plate being disposed on the surface of the corresponding lens on the front of the plate and vice versa. Since the projection screen comprises only one element, it is in principle inexpensive. As no Fresnel lens is used no Moire effect will occur. The two-dimensional lenticular structure on the rear of the screen concentrates the projected light in small focal points, so that a satisfactory contrast can be obtained by providing those surface portions of the lens through which no light emerges with an absorbing coating. The quality of this projection screen with respect to the uniformity of the brightness, color shifting, and color shading is slightly better than that of the afore-mentioned other projection systems but is not yet satisfactory. Moreover, forming the desired profiles in the moulds or rollers by means of which the projection screen is to be manufactured is very difficult, so that ultimately the projection screen is still very expensive.
Color shifting is defined as the change of color in the centre of the screen if the position of the viewer changes. Color shading is defined as the color difference between the center and the edges of the projection screen observed by a viewer whose line of view is perpendicular to the screen centre. Color shading occurs when lenses with large field-angles are used, which lenses are required if the projection system is to be accommodated in a cabinet of small volume.